feat: add cross-platform support via platform detection module

Introduces src/platform.rs with OS detection and env var overrides.
All hardcoded Linux paths replaced with Platform::detect() across
8 source files. Key changes:

- New Platform struct with 11 fields (all overridable via env vars)
- /proc/ access gated to Linux (#[cfg(target_os = "linux")])
- pgrep/pkill patterns broadened for cross-platform LS discovery
- sec-ch-ua-platform header now dynamic per OS
- Token, traces, config, CA cert paths use platform module
- LD_PRELOAD DNS redirect gated to Linux only
- Setup scripts for Linux (systemd) and macOS (launchd)
- find_ls_binary_path has cross-platform stubs

All 46 tests pass, cargo check clean.

scripts/setup-macos.sh

@@ -0,0 +1,65 @@
+#!/usr/bin/env bash
+# ZeroGravity — macOS setup
+# Installs a launchd plist for automatic startup and sets up config directories.
+# No UID isolation on macOS — runs in headless/HTTPS_PROXY mode only.
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+PROJECT_DIR="$(dirname "$SCRIPT_DIR")"
+CONFIG_DIR="$HOME/Library/Application Support/zerogravity"
+
+# ── 1. Config directory ──
+echo "→ Setting up config directory…"
+mkdir -p "$CONFIG_DIR"
+
+# ── 2. Data directory ──
+echo "→ Setting up /tmp/zerogravity-standalone…"
+mkdir -p /tmp/zerogravity-standalone
+
+# ── 3. Launchd plist ──
+echo "→ Installing launchd plist…"
+PLIST_DIR="$HOME/Library/LaunchAgents"
+PLIST="$PLIST_DIR/com.zerogravity.proxy.plist"
+mkdir -p "$PLIST_DIR"
+cat > "$PLIST" << EOF
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <key>Label</key>
+    <string>com.zerogravity.proxy</string>
+    <key>ProgramArguments</key>
+    <array>
+        <string>$PROJECT_DIR/target/release/zerogravity</string>
+    </array>
+    <key>WorkingDirectory</key>
+    <string>$PROJECT_DIR</string>
+    <key>EnvironmentVariables</key>
+    <dict>
+        <key>RUST_LOG</key>
+        <string>info</string>
+    </dict>
+    <key>KeepAlive</key>
+    <dict>
+        <key>SuccessfulExit</key>
+        <false/>
+    </dict>
+    <key>StandardOutPath</key>
+    <string>$HOME/Library/Logs/zerogravity.log</string>
+    <key>StandardErrorPath</key>
+    <string>$HOME/Library/Logs/zerogravity.log</string>
+</dict>
+</plist>
+EOF
+echo "  Installed: $PLIST"
+echo "  Start with: launchctl load $PLIST"
+echo "  Stop with:  launchctl unload $PLIST"
+
+# ── 4. Build ──
+echo "→ Building release binary…"
+cd "$PROJECT_DIR"
+cargo build --release 2>&1 | tail -1
+echo ""
+echo "✓ Setup complete."
+echo "  Start with: launchctl load $PLIST"
+echo "  Or manually: zg start"